The present invention relates to a rotor driving apparatus, and more particularly to a supporting portion for a driving apparatus in which a rotor as a rotating body can easily become imbalanced to cause large vibrations, such as a centrifugal separator.
In conventional rotor driving apparatuses such as centrifugal separators, the rotational torque obtained with a driving device such as an electric motor is transmitted to a rotor through a rotation shaft to thereby rotate the rotor. The rotor can be mounted with a plurality of test tubes each enclosing a sample, and centrifugal separation of the sample within each test tube is effected by the rotation of the rotor.
Examples of the rotors used in centrifugal separators include: an angle rotor in which angles of insertion holes, which are arranged at equal intervals and into which samples are inserted, are constant; and a swing rotor in which a container (referred to as “bucket”) to which the test tubes are mounted swings together with the rotation of the rotor. When performing a centrifugal operation, a user mounts test tubes to those rotors, each of the test tubes containing a sample for centrifugal separation. In this case, if the sample is contained in different amounts in the plural test tubes or if no test tube is inserted into a particular insertion hole, a center of gravity of the rotor and the test tubes as a whole is displaced from a center axis of the rotation, that is, eccentric gravity occurs so that the rotation of the rotor becomes imbalanced.
A rotational speed of a centrifugal separator is set in increments of 10 rpm in a range of, for example, from 300 to 1,000 rpm, and is set in increments of 100 rpm in a range of from 1,000 to the maximum rpm. In this case, a resonance point of a supporting system, which is determined based on a mass of the driving device and a spring constant of the supporting portion, may exist within its operating range. For instance, if an elastic shaft having a low rigidity is used as a rotation shaft, the elastic shaft has a large resonance point in a low-speed rotation region; once the resonance point is exceeded, a high-speed rotation can be attained in a stable manner.
When a rotor in an imbalanced state is rotated, the rotor generates vibrations, which are transmitted to the driving device or the casing. In particular, the vibrations become excessive near the above-mentioned resonance point, which often leads to breakage of the rotation shaft or the like. Thus, in order to suppress the vibrations of the driving device at the resonance point to a low level, a supporting portion having a vibration damping function is provided between the driving device and the casing. Generally, a supporting portion used for this purpose includes a spring element for blocking the transmission of vibrations to the casing, and a damper element such as a vibration isolation rubber for damping the vibrations. Therefore, in order to reduce the resonance magnification at the resonance point, the vibration isolation rubber selected should have a high energy-absorption factor (high loss factor).
However, the actual temperature of the vibration isolation rubber is not only dependent on room temperature (2 to 40° C.) at which it is used, but is also largely changed due to heat generated from an induction motor during driving. In that case, the damping characteristics of the rubber are changed to eliminate an initial high loss factor, which ultimately results in the generation of vibrations or noises in the apparatus.
For instance, assuming that a rotor is in the same unbalanced state, measurement of the rotor vibration amplitude is conducted with respect to the following two cases: a case where the temperature of the rubber is at the highest within the room temperature range in which the centrifugal separator can be used (when the loss factor and dynamic modulus of elasticity are at the minimum); and a case where the temperature of the rubber is at the lowest (when the loss factor and dynamic modulus of elasticity are at the maximum). The measured values are shown in FIG. 8. As indicated by a solid line A in a graph of FIG. 8, when the temperature of the vibration isolation rubber is at the highest, the amplitude at a first resonance point can be suppressed to a lower level in a low-speed rotation region. However, a sharp vibration peak appears in a range of 3,500 to 6,000 rpm, with the amplitude reaching its maximum level at the resonance point of a supporting system near 4,000 rpm. On the other hand, as indicated by a broken line B in FIG. 8, when the temperature of the vibration isolation rubber is at the lowest, a sharp vibration peak, such as one observed in the case where the temperature of the vibration isolation rubber is at the highest, does not appear in the range of 3,500 to 6,000 rpm. However, the amplitude at the first resonance point becomes extremely large in the initial low-speed rotation region. Note that the peak in the low rpm region refers to the first resonance point observed in the case where an elastic shaft having a low elasticity is used as the rotation shaft. In this case, the peak is inevitably exists within the operating range of the apparatus.